U.S. patent number 5,043,017 [Application Number 07/491,366] was granted by the patent office on 1991-08-27 for acid-stabilized calcium carbonate, process for its production and method for its use in the manufacture of acidic paper.
This patent grant is currently assigned to Pfizer Inc.. Invention is credited to June D. Passaretti.
United States Patent |
5,043,017 |
Passaretti |
August 27, 1991 |
Acid-stabilized calcium carbonate, process for its production and
method for its use in the manufacture of acidic paper
Abstract
A form of calcium carbonate, acid-stabilized by the addition to
finely divided calcium carbonate of one of a calcium- chelating
agent and a conjugate base, such as sodium hexametaphosphate,
followed by the addition of a weak acid, such as phosphoric acid,
is disclosed. A process for producing this material, and a method
for its use in the making of neutral to acidic paper in order to
improve the optical properties of the paper are also disclosed.
Inventors: |
Passaretti; June D. (Liberty
Corner, NJ) |
Assignee: |
Pfizer Inc. (New York,
NY)
|
Family
ID: |
23951903 |
Appl.
No.: |
07/491,366 |
Filed: |
March 9, 1990 |
Current U.S.
Class: |
106/465; 106/401;
106/464; 106/400; 106/499 |
Current CPC
Class: |
D21H
17/69 (20130101); C01F 11/185 (20130101); D21H
17/675 (20130101); C09C 1/021 (20130101); C01P
2004/62 (20130101); C01P 2006/12 (20130101); C01P
2004/61 (20130101); C01P 2004/11 (20130101) |
Current International
Class: |
C09C
1/02 (20060101); C01F 11/00 (20060101); C01F
11/18 (20060101); D21H 17/00 (20060101); D21H
17/69 (20060101); D21H 17/67 (20060101); C09C
001/02 () |
Field of
Search: |
;106/464,465,499 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Dixon, Jr.; William R.
Assistant Examiner: Hertzog; Scott L.
Attorney, Agent or Firm: Richardson; Peter C. Akers;
Lawrence C. Jaeger; Howard R.
Claims
What is claimed is:
1. An acid-stabilized finely divided calcium carbonate comprising a
mixture of at least about 0.1 weight percent of a compound selected
from the group consisting of a calcium-chelating agent and a
conjugate base, together with at least about 0.1 weight percent of
weak acid, with the balance to give 100 weight percent being finely
divided calcium carbonate, such that the calcium carbonate is
coated by and is in equilibrum with the calcium-chelating agent or
conjugate base and the weak acid.
2. The acid-stabilized finely divided calcium carbonate according
to claim 1 wherein the weak acid is selected from the group
consisting of phosphoric acid, metaphosphoric acid,
hexametaphosphoric acid, citric acid, boric acid, sulfurous acid,
acetic acid, and mixtures thereof.
3. The acid-stabilized finely divided calcium carbonate according
to claim 1 wherein the conjugate base is an alkali metal salt of a
weak acid.
4. The acid-stabilized finely divided calcium carbonate according
to claim 3 wherein the alkali metal is sodium or calcium.
5. The acid-stabilized finely divided calcium carbonate according
to claim 3 wherein the conjugate base is sodium
hexametaphosphate.
6. The acid-stabilized finely divided calcium carbonate according
to claim 1 wherein when the conjugate base is selected, the weak
acid part thereof and the weak acid are the same or are different
and are selected from the group consisting of phosphoric acid,
metaphosphoric acid, hexametaphosphoric acid, citric acid, boric
acid, sulfurous acid, acetic acid, and mixtures thereof.
7. The acid-stabilized finely divided calcium carbonate according
to claim 6 wherein the conjugate base is sodium hexametaphosphate
and the weak acid is phosphoric acid.
8. The acid-stabilized finely divided calcium carbonate according
to claim 1 wherein the calcium-chelating agent is selected from the
group consisting of sodium hexametaphosphate and common
multi-dentate chelating ligands.
9. The acid-stabilized finely divided calcium carbonate according
to claim 8 wherein the calcium-chelating agent is sodium
hexametaphosphate.
10. The acid-stabilized finely divided calcium carbonate according
to claim 8 wherein the common multi-denate chelating ligands are
selected from the group consisting of include ethylene diamine
tetra-acetic acid (EDTA), triethylene, tetramine, diethylene
triamine, o-phenanthroline, oxalic acid, and mixtures thereof.
11. The acid-stabilized finely divided calcium carbonate according
to claim 2 wherein the weak acid is phosphoric acid.
12. The acid-stabilized, finely divided calcium carbonate according
to claim 1 wherein the calcium carbonate is selected from the group
consisting of precipitated calcium carbonate and finely ground
natural limestone.
13. The acid-stabilized, finely divided calcium carbonate according
to claim 1 wherein the calcium-chelating agent or conjugate base is
from about 1 to about 2 weight percent of the final mixture.
14. The acid-stabilized finely divided calcium carbonate according
to claim 1 wherein the weak acid is from about 1 to about 4 weight
percent of the final mixture.
15. A process for producing an acid-stabilized finely divided
calcium carbonate comprising the steps of:
a) forming a mixture by adding to finely divided calcium carbonate,
at least about 0.1 weight percent, based on the dry weight of
calcium carbonate, of a compound selected from the group consisting
of a calcium-chelating agent and a conjugate base;
b) adding at least about 0.1 weight percent of a weak acid, based
on the dry weight of calcium carbonate, to the mixture to reduce
the pH of the resulting final mixture to from about 5 to about 7;
and
c) agitating the final mixture to ensure uniform mixing.
16. The process according to claim 15 wherein the weak acid is
selected from the group consisting of phosphoric acid,
metaphosphoric acid, hexametaphosphoric acid, citric acid, boric
acid, sulfurous acid, acetic acid, and mixtures thereof.
17. The process according to claim 15 wherein the calcium-chelating
agent is selected from the group consisting of sodium
hexametaphosphate and of common multi-dentate chelating
ligands.
18. The process according to claim 17 wherein the calcium-chelating
agent is sodium hexametaphosphate.
19. The acid-stabilized finely divided calcium carbonate according
to claim 8 wherein the common multi-dentate chelating ligands are
selected from the group consisting of include ethylene diamine
tetra-acetic acid (EDTA), triethylene tetramine, diethylene
triamine, o-phenanthroline, oxalic acid, and mixture thereof.
20. The process according to claim 15 wherein the conjugate base is
an alkali metal salt of a weak acid.
21. The process according to claim 20 wherein the alkali metal is
sodium or calcium.
22. The process according to claim 15 wherein when the conjugate
base is selected, the weak acid part thereof and the weak acid are
the same or are different and are selected from the group
consisting of phosphoric acid, metaphosphoric acid,
hexametaphosphoric acid, citric acid, boric acid, sulfurous acid,
acetic acid and mixtures thereof.
23. The process according to claim 20 wherein the conjugate base is
sodium hexametaphosphate.
24. The process according to claim 22 wherein the conjugate base is
sodium hexametaphosphate and the weak acid is phosphoric acid.
25. The process according to claim 15 wherein the finely divided
calcium carbonate is selected from the group consisting of a
precipitated calcium carbonate and finely ground natural
limestone.
26. The process according to claim 15 wherein the finely divided
calcium carbonate is in a form selected from the group consisting
of a dry powder and an aqueous slurry.
27. The process according to claim 26 wherein when the finely
divided calcium carbonate is a dry powder, the calcium-chelating
agent or conjugate base is in an aqueous solution and when the
finely divided calcium carbonate is an aqueous slurry, the
calcium-chelating agent or conjugate base is a solid or is in an
aqueous solution.
28. The process according to claim 15 wherein the weak acid is in a
form selected from the group consisting of pure liquid acid and an
aqueous solution thereof.
29. The process according to claim 15 wherein the calcium-chelating
agent or conjugate base is from about 1 to about 2 weight percent
of the final mixture.
30. The process according to claim 15 wherein the weak acid is from
about 1 to about 4 weight percent of the final mixture.
31. The acid stabilized finely divided calcium carbonate produced
according to the process of claim 15.
32. The acid-stabilized finely divided calcium carbonate according
to claim 8 wherein the common multi-dentate chelating ligands are
selected from the group consisting of include ethylene diamine
tetra-acetic acid (EDTA), triethylene tetramine, diethylene
triamine, o-phenanthroline, oxalic acid, and mixtures thereof.
Description
FIELD OF THE INVENTION
This invention relates to an acid-stabilized form of calcium
carbonate, a process for producing this material, and to a method
for use of the material as a filler material in the making of
neutral to weakly acidic paper to improve the optical properties of
the resulting paper.
BACKGROUND OF THE INVENTION
Titanium dioxide and calcined clay have traditionally been used as
filler materials in the making of neutral to weakly acidic paper,
to improve the optical properties of the resulting paper,
particularly its brightness. These materials, however, especially
titanium dioxide, have the disadvantage of being very expensive,
which results in a high cost of manufacturing the paper, and
consequently, the need to charge a high, uncompetitive price for
such paper.
Calcium carbonate, particularly precipitated calcium carbonate, is
used as a filler material in the making of alkaline paper. This
material results in a paper with enhanced optical properties.
Calcium carbonate is also significantly less expensive than
titanium dioxide, consequently there are appreciable economic
advantages to its use. Calcium carbonate, however, cannot generally
be used as a filler in acidic paper because it decomposes in an
acidic environment. Consequently, there has long been a need to
develop a calcium carbonate based material which is acid stabilized
and resistant to decomposition at low pH, so that it can be used as
a filler material in the manufacture of acidic paper, such as
groundwood paper, where use of an alkaline filler would have a
negative impact on the final paper properties.
Heretofore, the use of various phosphoric acids and their salts,
especially their sodium and potassium salts, in processes for the
production of calcium carbonate by the carbonation of an aqueous
suspension of calcium hydroxide, has been known.
Among the literature disclosing such processes are U.S. Pat. No.
4,240,870 wherein at least one of a phosphoric acid such as
orthophosphoric acid, metaphosphoric acid, hexametaphosphoric acid,
tripolyphosphoric acid, pyrophosphoric acid, tetrapolyphosphoric
acid and hexapolyphosphoric acid, or the sodium, potassium or zinc
salts thereof is added to an aqueous calcium hydroxide suspension
in the first step of a multi-step calcium carbonate production
process. The amount of acid or salt utilized is from about 0.01 to
about 5.0 weight percent of the calcium hydroxide in the
suspension.
Similarly, in U.S. Pat. No. 4,244,933, the reaction of the first
step or the second step of a multi-step calcium carbonate synthesis
is carried out in the presence of at least one of a phosphoric acid
and a water soluble salt thereof. The phosphoric acid or salt
thereof is chosen from among the same list, and utilized in the
same amount as given in the '870 patent, above.
U.S. Pat. No. 4,018,877 discloses the addition of a complex-forming
agent such as a polyphosphate, particularly sodium
hexametaphosphate, during the end of the first carbonation stage of
an multi-step calcium carbonate production process, preferably
after the bulk of the calcium carbonate precipitation has occurred,
or during the subsequent ageing or second carbonation step. The
amount of complexing agent used ranges from 0.001 to 5 weight
percent of the calcium carbonate produced.
U.S. Pat. No. 4,157,379 similarly discloses the addition of a
soluble metal salt such as an alkali metal phosphate, after primary
carbonation of an aqueous suspension of calcium hydroxide. The
amount of salt added is from about 0.001 to 0.5 mole percent of
calcium hydroxide in the starting suspension.
Published Japanese patent Application No. 090,821/60 discloses a
process for the preparation of calcium carbonate in which a
condensed phosphoric acid or its salt is added to a viscous
gelatinous emulsion formed by the carbonation of an aqueous calcium
hydroxide dispersion with a carbon dioxide-containing gas. The
condensed phosphoric acid may be hexametaphosphoric,
pyrophosphoric, tripolyphosphoric, polyphosphoric or
ultraphosphoric acid.
Published Japanese Patent Application No. 090,822/60 discloses the
same basic process as is disclosed in published Japanese Patent
Application No. 090,821/60, above, but further including the
presence of a magnesium-containing compound in the aqueous calcium
hydroxide dispersion.
In none of the foregoing references, however, is it disclosed or
suggested that the phosphoric acid or salt thereof added during
preparation of the calcium carbonate has the effect of making the
resulting calcium carbonate product acid-resistant. Moreover, in
all of the above processes, the acid or salt addition is to the
calcium hydroxide suspension prior to or during carbonation or to
the calcium carbonate precursor just after precipitation, rather
than to the final calcium carbonate particles.
U.S. Pat. No. 4,793,985 discloses the addition of from 0.2 to 0.4
weight percent of a dispersing agent such as water soluble salts of
polyphosphoric acid or phosphates, particularly, sodium
hexametaphosphate, to a slurry of ground calcium carbonate, in
order to improve solids distribution within the liquid, as part of
a process for producing an ultrafine calcium carbonate with an
average particle size of less than 2 microns.
Although the above reference involves the phosphoric acid or
polyphosphate addition to a calcium carbonate, the addition is to
the calcium carbonate in slurry form and nothing is disclosed or
suggested about the acid or phosphate rendering the resulting
ultrafine calcium carbonate acid-resistant.
The use of polyphosphoric acid and polyphosphates as dispersants or
surfactants in slurries of mineral particles, such as calcium
carbonate, for use in waste treatment, is disclosed in U.S. Pat.
No. 4,610,801.
U.S. Pat. No. 4,219,590 discloses the treatment of calcium
carbonate particles of not more than 20 microns average particle
diameter with an acid gas capable of reacting with the calcium
carbonate, such as the acid gas obtained by heating phosphoric
acid, in order to finely uniformize the calcium carbonate particle
size and coat the particle surface with the calcium salt of the
acid gas. This reference further discloses that when the acid gas
is hydrogen fluoride, sulfur dioxide, phosphoric anhydride or a
chloride or fluoride of titanium, aluminum or silica, the resulting
calcium carbonate particles demonstrate reduced solubility in
acids. The process is based on a solid-gaseous phase contact in a
fluidized bed type reactor. In utilizing a gaseous contact process,
the patent suggests, however, that there are inherent drawbacks to
utilizing a method wherein surface treatment of the calcium
carbonate is effected by treating an aqueous suspension of calcium
carbonate with a solution or emulsion of the surface treatment
agent.
Japanese Patent No. 030,812/82 discloses a method for improving the
surface of calcium carbonate particles using an aqueous solution of
a condensed phosphate, only, such as a metaphosphate or
pyrophosphate, which is added to an aqueous calcium carbonate
suspension. The method gives calcium carbonate particles an acid
resistance and reduces the pH of the particles by 0.1-5.0.
SUMMARY OF THE INVENTION
Accordingly, a form of calcium carbonate which is acid-stabilized
to enable its use as a filler material in the making of neutral to
weakly acidic paper, and a process for producing this acid-stable
calcium carbonate, based on the addition of a compound which is a
calcium-chelating agent and/or a conjugate base, followed by a weak
acid, to finely divided calcium carbonate in a solid-liquid
reaction and coating process, have been discovered. A preferred
calcium-chelating agent or conjugate base and weak acid pair is
sodium hexametaphosphate and phosphoric acid.
The acid-stabilized form of calcium carbonate produced according to
the present invention has been found to be particularly effective
as a filler material in papermaking in that it produces a paper
with enhanced optical properties, especially, improved pigment
scattering coefficient, opacity, and brightness. The
acid-stabilized calcium carbonate of this invention is also
economical and significantly less expensive than previously used
titanium dioxide and calcined clay fillers. The material prepared
according to this invention is also useful as a pigment in
paint.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the pH stability over time of precipitated calcium
carbonate which has been acid-stabilized with sodium
hexametaphosphate and phosphoric acid.
FIG. 2 shows the final pH after extended agitation of precipitated
calcium carbonate acid-stabilized with varying levels of sodium
hexametaphosphate and 6 weight percent phosphoric acid.
FIG. 3 shows the final pH after extended agitation of precipitated
calcium carbonate acid-stabilized with varying levels of sodium
hexametaphosphate and phosphoric acid.
FIG. 4 shows the pH stability over time of calcium carbonate in the
form of fine ground limestone which has been acid-stabilized with
sodium hexametaphosphate and phosphoric acid and with phosphoric
acid only.
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS OF THE INVENTION
Acid-stable calcium carbonate is a form of calcium carbonate that
is stable in a mildly acidic environment. The ability of
acid-stable calcium carbonate to resist degradation in a mildly
acidic environment is due to a buffering action between an absorbed
or reacted calcium-chelating agent or a conjugate base on the
surface of the calcium carbonate and a weak acid in solution.
Without wishing to be limited to a particular theory, it is
believed that the calcium-chelating agent or conjugate base, when
applied to the surface of the calcium carbonate, acts to reduce the
solubility of the surface of the calcium carbonate.
The anions of the calcium-chelating agent or the conjugate base and
the anions of the weak acid, may be the same, although it is not
required that they be the same.
For example, when sodium hexametaphosphate is added to calcium
carbonate as the conjugate base, the weak acid may be any weak acid
such as phosphoric or sulfurous acid, with phosphoric acid being
preferred.
The buffered, acid-stable calcium carbonate of the present
invention can be any weak acid/conjugate base system, such as
citric acid/citrate, sulfurous acid/ sulfite, boric acid/borate,
and combinations thereof.
According to the present invention, calcium carbonate is
acid-stabilized by mixing therewith at least about 0.1 weight
percent of a calcium-chelating agent or a conjugate base and at
least about 0.1 weight percent of a weak acid. The
calcium-chelating agent is a compound selected from the group
consisting of sodium hexametaphosphate, which is preferred, and
common multi-dentate chelating ligands, including ethylene diamine
tetra-acetic acid (EDTA), triethylene tetramine, diethylene
triamine, o-phenanthroline, oxalic acid and the like. The conjugate
base of the present invention is an alkali metal salt of a weak
acid. Preferably, the alkali metal is sodium or calcium. Sodium
hexametaphosphate, in addition to being a calcium-chelating agent,
is also a conjugate base, and is a preferred example of a conjugate
base in the present invention as well. Some embodiments of the weak
acid are phosphoric acid, metaphosphoric acid, hexametaphosphoric
acid, citric acid, boric acid, sulfurous acid, acetic acid and
mixtures thereof. Phosphoric acid is preferred.
Preferably, the calcium-chelating agent or conjugate base is from
about 1 to about 2 weight percent of the final mixture, and the
weak acid is from about 1 to about 4 weight percent of the final
mixture.
The calcium carbonate is preferably finely divided and it can be
either a precipitated calcium carbonate or a natural ground
limestone.
According to one embodiment of the process of the present
invention, the calcium-chelating agent or conjugate base is first
mixed with the finely divided calcium carbonate. The weak acid is
then added to the mixture and the combined mixture is agitated for
a sufficiently long period of time to ensure uniform mixing of the
ingredients.
In an alternative embodiment of the process, the calcium-chelating
agent or the conjugate base is mixed with the finely divided
calcium carbonate. The weak acid is provided in a paper furnish
with which the calcium carbonate-containing mixture, as the filler
material, is then combined and further mixed during the papermaking
process.
In a further alternative embodiment of the process, both the
calcium-chelating agent or the conjugate base and the weak acid are
provided in a paper furnish which is added to the calcium carbonate
during the paper making process.
The components of the acid stabilized calcium carbonate, including
the calcium-chelating agent or conjugate base and the weak acid,
can be dynamically added to a calcium carbonate-containing paper
mixture in a paper making machine as part of a paper furnish, at
various times in the course of the papermaking process.
In the embodiment of the process of the present invention wherein
both the calcium-chelating agent or conjugate base and weak acid
are provided in the paper furnish, it is important that the
calcium-chelating agent or conjugate base be sequentially fed to
the calcium carbonate-containing mixture before the weak acid.
In all three alternative embodiments, a preferred pair of
calcium-chelating agent or conjugate base and weak acid is sodium
hexametaphosphate and phosphoric acid.
For any embodiment of the process, the finely divided calcium
carbonate may be in a form either as a dry powder or an aqueous
slurry with up to about 60 weight percent solids content.
The calcium-chelating agent or conjugate base can be utilized in a
form either as a solid or as an aqueous solution. It has been found
that when the finely divided calcium carbonate is in dry powder
form, it is preferable to utilize an aqueous solution of the
calcium-chelating agent or conjugate base, rather than the solid
form, in order to facilitate homogeneous mixing. Where the calcium
carbonate is in the form of an aqueous slurry, the solid form of
the calcium-chelating agent or conjugate base readily dissolves
therein and is the preferred form if it is desired to minimize the
overall volume of the mixture
The weak acids utilizable according to the invention may be
utilized in pure concentrated form or as an aqueous solution.
It has been found that, according to the process of the present
invention, the level of addition of the calcium-chelating agent or
conjugate base needed to acid-stabilize the calcium carbonate is as
low as about 0.1 weight percent, on a total final mixture weight
basis. A calcium-chelating agent or conjugate base content of from
about 1 to about 2 weight percent, on a total final mixture weight
basis, is preferred.
Similarly, it has been found that, according to the process of the
present invention, the level of weak acid addition needed to
stabilize the calcium carbonate is also as low as about 0.1 weight
percent, on a total final mixture weight basis. A weak acid content
of from about 1 to about 4 weight percent, on a total final mixture
weight basis, is preferred.
Where a conjugate base is utilized, the weak acid may be the same
acid as the week acid part of the conjugate base or it may be
different.
For example, it has been found that a preferred conjugate base/weak
acid pair according to the invention is sodium hexametaphosphate
and phosphoric acid.
The polyphosphate backbone of the hexametaphosphate exhibits a
sequestering action which enables the hexametaphosphate to react
with the calcium carbonate surface thereby lowering the solubility
of the calcium carbonate. When phosphoric acid is added to calcium
carbonate, initially the pH of the slurry is lowered to
approximately 5.0. However, within a few minutes of agitation, the
pH rises to 8.0. The species that are believed to be formed when
the phosphoric acid is added to calcium carbonate are Ca(H.sub.2
PO.sub.4).sub.2, CaHPO.sub.4 and Ca.sub.3 (PO.sub.4).sub.2. These
three species are in equilibrium with one another, however, their
solubilities decrease in the order Ca(H.sub.2
PO.sub.4)>CaHPO.sub.4 >Ca.sub.3 (PO.sub.4).sub.2. When
CaHPO.sub.4 forms, it precipitates out of solution which pushes the
equilibrium towards its formation. Eventually all the phosphoric
acid is converted to CaHPO.sub.4 or Ca.sub.3 (PO.sub.4).sub.2,
calcium carbonate disassociates and the pH rises. Brushite,
CaHPO.sub.4, can be detected in the samples via powder x-ray
diffraction.
When sodium hexametaphosphate is added to the calcium carbonate
slurry, it chelates with the Ca+2 that is on the surface of calcium
carbonate and in solution. When H.sub.3 PO.sub.4 is added, it forms
Ca(H.sub.2 PO.sub.4).sub.2 but the pH of the solution stays acidic
due to the H+ formed by the hydrolysis of the (NaPO.sub.3).sub.6.
As more CaCO.sub.3 dissolves, the Ca+2 concentration increases to
the point where the system reaches equilibrium and no more
CaCO.sub.3 can disassociate. The equilibrium pH is acidic and is
dependant upon the amount of sodium hexametaphosphate and
phosphoric acid added.
The above-described three component system consists of calcium
carbonate, a calcium-chelating agent or a conjugate base, such as
sodium hexametaphosphate, and a weak acid, such as phosphoric acid.
Any calcium-chelating agent or conjugate base and weak acid are
usable in this system, but sodium hexametaphosphate and phosphoric
acid are preferred. Calculations show that theoretically about 0.1
percent of hexametaphosphate is needed per 1 m.sup.2 /g of calcium
carbonate surface area for complete coverage of the surface,
however, in actuality, two to three times the theoretical minimum
is necessary for complete coverage, depending on the final amount
of weak acid that is added. This is evident in the following
examples. As a more practical means of measuring the amounts of
hexametaphosphate and phosphoric acid added, however, the amounts
typically are expressed as a percent by weight of the dry calcium
carbonate.
The nature and scope of the present invention may be more fully
understood in view of the following non-limiting examples, which
demonstrate the effectiveness of a buffered system using sodium
hexametaphosphate and phosphoric acid.
All calcium carbonates utilized in the following examples were
precipitated from Pfizer lime from the Adams, Mass. limestone
quarry or were undispersed fine ground limestone also from the
Adams, MA limestone quarry. All of the precipitated calcium
carbonates used in the following examples were prismatic in
morphology, with surface area of 7-11 m.sup.2 /g and average
particle sizes of 0.7 to 1.4 microns. The pH of all precipitated
calcium carbonate slurries was adjusted to 8.0 using carbon
dioxide.
EXAMPLE 1
Effectiveness of the Buffer System
Previously, there have been some attempts at making calcium
carbonate stable in mildly acidic environments by adding a weak
acid to calcium carbonate. Initially, the pH of the system may be
below 6.0, but with agitation, the pH quickly rises above 8.0.
However, when sodium hexametaphosphate is added to the calcium
carbonate prior to the addition of phosphoric acid, the pH of the
system remains acidic. When 6% of phosphoric acid, based on the dry
weight of calcium carbonate, was added to one liter of a 15% solids
slurry of precipitated calcium carbonate, the pH initially was
reduced to 5.2. When the sample was agitated, the pH of the slurry
immediately increased to 8.0. However, when 1% by weight of sodium
hexametaphosphate, based on the dry weight of calcium carbonate,
was added prior to the phosphoric acid, the pH of the slurry only
increased to 5.4 upon agitation. The pH stability of precipitated
calcium carbonate with sodium hexametaphosphate and phosphoric acid
was compared to the pH stability of precipitated calcium carbonate
with phosphoric acid, as shown in FIG. 1.
EXAMPLE 2
Effectiveness of the Buffer System
A 15% by weight solids slurry of prismatic calcium carbonate was
divided into seven 4-liter portions to which from 0 to 6 percent by
weight of sodium hexametaphosphate, based on the dry weight of
calcium carbonate, was added in 1 percent increments, followed by
the addition of 6% of phosphoric acid, based on the dry weight of
calcium carbonate. The materials were then agitated for 18 hours to
determine the acid stability. After 18 hours, the samples that had
been treated with phosphoric acid only had reached a final pH of
8.2, whereas the samples that had been treated with sodium
hexametaphosphate prior to phosphoric acid addition had final pH's
below 6.5. This can be seen in FIG. 2.
EXAMPLE 3
Determination of Additive Level
To determine the amount of additive necessary to obtain a final
acidic pH range, from 0 to 6% of sodium hexametaphosphate, based on
the dry weight of calcium carbonate, was added to seven 3000 ml
portions of a 15% by weight solids slurry of prismatic calcium
carbonate. Each of the 3000 ml portions was then split into six 500
ml aliquots into which from 1-6% of phosphoric acid, based on the
dry weight of calcium carbonate, was added in 1 percent increments.
The materials were agitated and the pH was measured after 18 hours
of agitation. FIG. 3 is a plot of the pH measured for each sample
after 18 hours. From FIG. 3, it can be seen that an addition of 2%
of sodium hexametaphosphate, based on the dry weight of calcium
carbonate, followed by the addition of 6% of phosphoric acid, based
on the dry weight of calcium carbonate, resulted in a calcium
carbonate slurry that had a stable pH of 5.3. The levels of sodium
hexametaphosphate and phosphoric acid required to be added to give
a final desired pH are also determinable from FIG. 3.
EXAMPLE 4
Acid Stable Ground Limestone
Undispersed fine ground limestone from the Pfizer Adams limestone
quarry in Mass. was made into a 15% by weight solids slurry. The pH
of the slurry was 9.2. To 2000 ml of this slurry, sodium
hexametaphosphate was added (1% by weight based on dry calcium
carbonate), followed by an addition of phosphoric acid (2% by
weight based on dry calcium carbonate). The pH of this slurry,
initially and after 24 hours of agitation was compared to the pH of
2000 ml of a 15% by weight solids slurry of the same ground
limestone to which 2% of phosphoric acid, based on the dry weight
of calcium carbonate, was added. Initially, the pH's of both
slurries was 5.5-5.7. However, after 24 hours, the pH of the slurry
that was reacted with sodium hexametaphosphate and phosphoric acid
was 6.0, whereas the pH of the slurry that had only phosphoric acid
addition was 8.0. This is shown in FIG. 4.
* * * * *